Aromatic hydrocarbon compounds such as benzene are frequently used for producing transportation fuels and petrochemicals such as styrene, phenol, nylon, polyurethanes and many others. Benzene can be produced, e.g., by steam cracking and naphtha reforming. During steam cracking, a C2+ hydrocarbon feed reacts in the presence of steam under high-temperature pyrolysis conditions to produce a product comprising molecular hydrogen, C4-olefin, other C4-hydrocarbon, and C5+ hydrocarbon. The yield of aromatic hydrocarbon from steam cracking is generally much less than the yield of light hydrocarbon, and processes of significant complexity are typically needed for aromatics separation and recovery. Naphtha reforming catalytically produces a product having a much greater content of aromatic hydrocarbon than does steam cracker effluent, but the naphtha feed is itself useful for other purposes such a as motor gasoline blendstock.
Attempts have been made to overcome these difficulties, and provide an efficient process for producing aromatic hydrocarbon at high yield from a relatively inexpensive feed. For example, processes have been developed for producing light aromatic hydrocarbon (e.g., benzene, toluene, and xylenes—“BTX”) from paraffinic C1-C4 feeds. The processes typically utilize a catalyst having a molecular sieve component e.g., ZSM-5, and a dehydrogenation component, such as one or more of Pt, Ga, Zn, and Mo. These conventional processes typically operate at high temperature and low pressure. Although these conditions increase the yield of aromatic hydrocarbon, they also lead to an increased rate of catalyst deactivation, mainly resulting from increased catalyst coking.
One way to lessen the amount of catalyst coking involves increasing the relative amount of methane in the feed, as disclosed in U.S. Pat. No. 5,026,937. The relative amount of methane can be increased by removing C2+ hydrocarbon from the feed. Since ethane, propane, and butanes are less refractory than methane, removing these compounds from the feed decreases the amount of over-cracking, and lessens the accumulation of catalyst coke.
Aromatization processes having a decreased selectivity for catalyst coke have also been developed. For example, U.S. Pat. No. 4,855,522 discloses using a dehydrocyclization catalyst comprising (a) an aluminosilicate having a silica : alumina molar ratio of at least 5 and (b) a compound of (i) Ga and (ii) at least one rare earth metal. The aromatization is carried out at a temperature ≧450° C. (e.g., 475° C. to 650° C.) and a pressure of from 1 bar to 20 bar. Other processes limit decrease selectivity for catalyst coke by carrying out the reaction for a relatively short time (e.g., less than a day), and then halting the reaction so that the catalyst can be regenerated. For example, U.S. Patent Application Publication No. 2009/0209794 A1, and U.S. Pat. Nos. 8,692,043 and 9,144,790 disclose a process for aromatizing lower alkanes such as ethane using a particulate catalyst, where the catalyst particles have an average catalyst particle residence time in the reaction zone in the range of about 0.1 second to about 30 minutes. According to those references, such a residence time can be achieved by carrying out the aromatization in a fluid bed, and continuously withdrawing catalyst from the bed for regeneration. Maximum ethane conversion is about 63%, and the catalyst and process conditions which achieve appreciable ethane conversion also exhibit appreciable selectivity for methane.
It is desired to produce aromatic hydrocarbon from saturated hydrocarbon at greater feed conversion, particularly with less methane yield. Processes which operate at a space velocity (GHSV) greater than 1000 hr−1 are particularly desired, as are those which can utilize a feed comprising relatively refractory hydrocarbon such as methane.